Several solutions have been investigated for obtaining a wide dynamic range. First of all, there exist solutions using a single sensor equipped with means for image capture with high dynamic range. These systems use various techniques, amongst which is the successive capture of several images with various integration times. If the signal supplied by a pixel of the sensor having undergone a long integration time is saturated, it is replaced using a signal from the same pixel that has undergone a shorter integration time. This pre-supposes that several successive images are taken thus considerably increasing the overall acquisition time and not allowing real time capture of a sequence of images.
Another technique consists in having a mixed matrix with small pixels and large pixels. The small, less sensitive, pixels are used where there is a large amount of light. A complex processing operation is needed, which reduces the overall resolution of the matrix, notably that of the poorly lit regions (only exciting the large pixels).
Another technique consists in measuring the time that a pixel takes to reach saturation in order to deduce from this information on the light level in the presence of saturating illumination. This pre-supposes the use of a “complex” pixel technology that requires “large pixels” consequently resulting in a loss of resolution.
Techniques with pixels using a logarithmic or linear-logarithmic function or a variation in slope of the response curve have furthermore been proposed for pixels with three or more transistors. They are based on a variation of the potential of the gate of the reset transistor for the photodiode. These solutions are sensitive to technological dispersions, namely the dispersion in threshold voltages of the transistors of the various pixels and the dispersion of the dark potential of the photodiode after resetting. They do not allow the dynamic range of the device to be greatly increased (100 dB maximum), increase the spatial noise and involve complex post-processing owing to the modification of the linearity of the grey levels.
The majority of these techniques, except for the hybridization of pixels of different sizes, have the drawback of degrading the signal-to-noise ratio of the dark regions.
Solutions also exist comprising two cameras disposed adjacent to one another, operating with different integration times, and also comprising means for reconstruction of an image common to the two cameras. These solutions have the drawback of requiring a pre-processing phase for the images coming from the two cameras before carrying out the correlation needed for the establishment of an image with high dynamic range. The pre-processing increases the latency time of the device and the correlation degrades the resolution of the overall image owing to the correlation defects of the system and to the possible presence of objects at close distances causing problems of parallax.